Making Soap at Home in the UK: A Beginner's Guide to Cold Process

Making your own soap sounds like witchcraft until you've done it once, and then it feels almost too simple. You melt some oils, mix a lye solution, blend the two together, pour it into a mould and wait. That's very nearly all of it. The catch, and it's an important one, is the lye. Let's get the chemistry and the safety straight first, then get you to your first bar.

How does soap actually get made?

Soap is a chemical reaction, not a recipe you can fudge. When you combine fats or oils with lye, they react and turn into two new things: soap and glycerin. That reaction is called saponification, and it's the whole game. The oils bring the skin-loving qualities, the lye does the transforming, and once they've fully reacted, neither exists in its raw form any more. What's left is a gentle bar with a natural dose of glycerin, which is why homemade soap feels so much kinder than a lot of shop bars.

For solid bar soap the lye is sodium hydroxide, sometimes labelled NaOH or caustic soda. Liquid soap uses potassium hydroxide instead, but stick with bars while you're learning.

There are two beginner-friendly ways to make soap. Cold process, which we're covering here, is making it properly from scratch: you handle the lye yourself and you get complete control over what goes in. Melt and pour is the other route, where you buy a ready-made soap base, melt it down, add colour and scent, and set it. Melt and pour is brilliant for kids' projects and quick gifts, but the soap chemistry was done for you at a factory. Cold process is where the real craft lives.

Is there such a thing as lye-free soap?

No, and it's worth being clear about this because the internet muddies it constantly. You cannot make soap without lye. Soap is, by definition, the result of the lye-and-oil reaction. If there was no lye involved at any point, it isn't soap. It's a "syndet" bar, a detergent, or something else entirely.

When a product is sold as "lye-free" or "no lye" soap making, that's melt and pour. The lye was used to create the base, then fully reacted away, before it was sold to you. So you're not avoiding lye, you're just buying it pre-reacted.

There's no soap without lye. The lye is fully consumed as it works, so a properly made, cured bar contains none of it at all.

Here's the reassuring bit. Because the lye is completely used up during saponification, a correctly made and cured bar has no lye left in it whatsoever. The scary ingredient does its job and vanishes. All the caution below is about handling raw lye during the making, not about the finished soap sitting in your bathroom.

How dangerous is lye, and how do I stay safe?

Lye deserves your full respect. In its raw form it's strongly caustic and will burn skin, damage eyes and irritate your lungs if you breathe the fumes. It also heats up dramatically the moment it hits water, easily reaching 80 to 90°C in seconds. None of this should put you off, but all of it means you follow the rules every single time.

Use dedicated equipment once you start, kept separate from your cooking kit. Stainless steel and heavy-duty plastic are fine for lye. Aluminium is not, because lye reacts with it and gives off hydrogen gas. Take your time, don't make soap when you're tired or distracted, and you'll be perfectly safe.

Why must I use a soap calculator?

This is the part beginners most want to skip, and the one you absolutely cannot. Every oil needs a different amount of lye to turn into soap. Coconut oil, olive oil and shea butter each have their own "saponification value", the precise amount of lye it takes to convert them. So a lye quantity that's perfect for one recipe will be wildly wrong for another.

Get too little lye and the bar stays soft and greasy. Get too much and you've made a lye-heavy, caustic bar that can burn skin. Neither is what you want, and you cannot eyeball it.

A soap calculator solves this in seconds. You type in your oils and their weights, it works out exactly how much lye and water you need, and it lets you set your superfat. Superfat means deliberately using slightly less lye than would react with all the oils, so a small amount of oil stays free in the bar to keep it gentle. For a beginner, 5% is the sweet spot. Free tools like SoapCalc or the Bramble Berry Lye Calculator are the industry standard, and every serious soaper uses one for every batch. Treat any recipe you find online, including the framework below, as a starting point to run through the calculator, not gospel.

What kit and ingredients do I need to start?

The lovely thing about cold process is how little you need. Most of it's already in a British kitchen.

For kit: accurate digital scales that measure to the gram, a stick blender (this is the one real timesaver, doing in minutes what used to take an hour by hand), a heatproof stainless steel or plastic jug for the lye solution, a couple of mixing bowls, a spatula, a thermometer, and a mould. A silicone loaf mould or even a clean lined tub works perfectly for your first go. Plus the safety gear: goggles, gloves, long sleeves.

For ingredients: sodium hydroxide (buy soap-grade from a UK supplier), distilled or cooled boiled water, and your oils. A classic gentle starter blend is something like olive oil, coconut oil and a butter or a hardening oil, which gives you a mild bar with decent lather and a firm set. Add a skin-safe essential oil or fragrance oil if you fancy scent.

As a representative example only, a common beginner loaf runs along the lines of roughly 60% olive oil, 25% coconut oil and 15% shea butter or similar, with the lye and water calculated to a 5% superfat. Do not weigh out lye from that percentage in your head. Put your exact oils and total weight into a soap calculator and use the numbers it gives you.

How do I make a batch, step by step?

Once you're geared up and your recipe's calculated, the process itself is calm and quick. Read all the steps through before you start.

1. 1

   Prep and weigh

   Put on your goggles and gloves. Weigh out your oils into one bowl, and separately weigh your water and your lye into their own containers, using the exact figures from your soap calculator.

2. 2

   Make the lye solution

   By an open window, slowly add the lye to the water (never the reverse), stirring gently. It'll heat up fast and give off fumes, so keep your face back. Set it aside somewhere safe to cool to around 40 to 50°C.

3. 3

   Melt the oils

   Gently melt any solid oils and butters, then combine with your liquid oils. Aim to bring them to a similar temperature to your lye solution, roughly 40 to 50°C.

4. 4

   Combine and blend to trace

   Carefully pour the lye solution into the oils. Use your stick blender in short bursts until the mixture thickens to 'trace', the point where a drizzle leaves a faint trail on the surface, like thin custard.

5. 5

   Add scent and pour

   Quickly stir in your essential or fragrance oils, then pour the batter into your mould. Tap it down to knock out air bubbles and smooth the top.

6. 6

   Insulate and set

   Cover the mould and leave it somewhere warm and undisturbed for 24 to 48 hours, until the soap is firm enough to unmould and cut into bars.

That's it for the making. Everything from here on is patience.

Why does soap need to cure, and can I sell it?

Your bars are firm within a day or two, but they are not ready. Cold process soap needs to cure for four to six weeks. During curing, excess water slowly evaporates, so the bar hardens up and lasts far longer in the shower. The soap also mellows and becomes milder as any last traces of the reaction settle. Skipping the cure gives you a soft, short-lived bar that's harsh on the skin, so this genuinely is worth the wait. Stand your bars on a rack with air circulating all around, turn them now and then, and label them with the date.

For personal use, gifts and family, that's the entire story. Making soap at home for yourself is completely unregulated in the UK, so go and enjoy it.

Selling is a different matter. In the UK, soap for washing skin is legally a cosmetic, and the rules make no exception for tiny kitchen operations. Before you sell a single bar, each product (technically each recipe) needs a Cosmetic Product Safety Report, or CPSR, written and signed by a suitably qualified safety assessor. You then have to register the product on the government's Submit Cosmetic Product Notification (SCPN) service and name a UK Responsible Person on your labelling. Skipping this isn't a grey area: failing to notify can carry a fine and up to three months in prison. It's very doable, plenty of small UK soapers go through it, but budget the time and cost before you plan a market stall.

Ready for more? Browse the rest of our Natural Home guides, or build up the wider know-how over on our Skills hub.